Essential Nutrients for Plant Growth: Nutrient Functions and Deficiency Symptoms

Home , Chlorosis, Nitrogen deficiency, Nutrient management, Phosphorus deficiency

Plant Nutrient Management in Hawaii’s Soils From: Plant Nutrient Management in Hawaii’s Soils, Approaches for Tropical and Subtropical Agriculture J. A. Silva and R. Uchida, eds. College of Tropical Agriculture and Human Resources, University of Hawaii at Manoa, ©2000

Chapter 3

Essential Nutrients for Plant Growth: Nutrient Functions and Deficiency Symptoms

R. Uchida

lants, like all other living things, need food for their their function in plants, symptoms of their deficien- Pgrowth and development. Plants require 16 essen- cies, and recommended nutrient levels in plant tissues tial elements. Carbon, hydrogen, and oxygen are de- of selected crops. rived from the atmosphere and soil water. The remain- ing 13 essential elements (nitrogen, phosphorus, po- Nitrogen Ð tassium, calcium, magnesium, sulfur, iron, zinc, man- symbol: N; available to plants as nitrate (NO3 ) , and + ganese, copper, boron, molybdenum, and chlorine) are ammonium (NH4 ) ions. supplied either from soil minerals and soil organic Nutrient functions matter or by organic or inorganic fertilizers. N is biologically combined with C, H, O, and S to For plants to utilize these nutrients efficiently, light, ¥ create amino acids, which are the building blocks of heat, and water must be adequately supplied. Cultural proteins. Amino acids are used in forming proto- practices and control of diseases and insects also play plasm, the site for cell division and thus for plant important roles in crop production. growth and development. Each type of plant is unique and has an optimum Since all plant enzymes are made of proteins, N is nutrient range as well as a minimum requirement level. ¥ needed for all of the enzymatic reactions in a plant. Below this minimum level, plants start to show nutri- N is a major part of the chlorophyll molecule and is ent deficiency symptoms. Excessive nutrient uptake can ¥ therefore necessary for photosynthesis. also cause poor growth because of toxicity. Therefore, N is a necessary component of several vitamins. the proper amount of application and the placement of ¥ N improves the quality and quantity of dry matter in nutrients is important. ¥ leafy vegetables and protein in grain crops. Soil and plant tissue tests have been developed to assess the nutrient content of both the soil and plants. Deficiency symptoms (p. 33) By analyzing this information, plant scientists can de- ¥ Stunted growth may occur because of reduction in termine the nutrient need of a given plant in a given cell division. soil. In addition to the levels of plant-available nutri- ¥ Pale green to light yellow color (chlorosis) appear- ents in soils, the soil pH plays an important role in nu- ing first on older leaves, usually starting at the tips. trient availability and elemental toxicity (see p. 46). Depending on the severity of deficiency, the chloro- This chapter describes the essential nutrients, the sis could result in the death and/or dropping of the chemical forms in which they are available to plants, older leaves. This is caused by the translocation of

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N from the older to the younger tissues. ¥ K assists in regulating the plant’s use of water by ¥ Reduced N lowers the protein content of seeds and controlling the opening and closing of leaf stomates, vegetative parts. In severe cases, flowering is greatly where water is released to cool the plant. reduced. ¥ In photosynthesis, K has the role of maintaining the ¥ N deficiency causes early maturity in some crops, balance of electrical charges at the site of ATP pro- which results in a significant reduction in yield and duction. quality. ¥ K promotes the translocation of photosythates (sugars) for plant growth or storage in fruits or roots. Phosphorus ¥ Through its role assisting ATP production, K is in- symbol: P; available to plants as orthophosphate ions volved in protein synthesis. 2 Ð Ð (HPO4 , H2PO4 ). ¥ K has been shown to improve disease resistance in plants, improve the size of grains and seeds, and im- Nutrient functions prove the quality of fruits and vegetables. ¥ In photosynthesis and respiration, P plays a major role in energy storage and transfer as ADP and ATP Deficiency symptoms (p. 35) (adenosine di- and triphosphate) and DPN and TPN ¥ The most common symptom is chlorosis along the (di- and triphosphopyridine nucleotide). edges of leaves (leaf margin scorching). This occurs ¥ P is part of the RNA and DNA structures, which are first in older leaves, because K is very mobile in the the major components of genetic information. plant. ¥ Seeds have the highest concentration of P in a ma- ¥ Because K is needed in photosynthesis and the syn- ture plant, and P is required in large quantities in thesis of proteins, plants lacking K will have slow young cells, such as shoots and root tips, where me- and stunted growth. tabolism is high and cell division is rapid. ¥ In some crops, stems are weak and lodging is com- ¥ P aids in root development, flower initiation, and seed mon if K is deficient. and fruit development. ¥ The size of seeds and fruits and the quantity of their ¥ P has been shown to reduce disease incidence in some production is reduced. plants and has been found to improve the quality of . . . continued on p. 49 certain crops. Deficiency symptoms (p. 34) ¥ Because P is needed in large quantities during the early stages of cell division, the initial overall symptom is slow, weak, and stunted growth. —Color Section— ¥ P is relatively mobile in plants and can be transferred Symptoms of plant nutrient to sites of new growth, causing symptoms of dark to deficiencies and toxicities blue-green coloration to appear on older leaves of some plants. Under severe deficiency, purpling of Photo credits leaves and stems may appear. Abbreviations given for sources of the photographs are ¥ Lack of P can cause delayed maturity and poor seed as follows: and fruit development. ASHS: From Avocado Production in California, a slide set produced by Eugene Memmler and formerly distrib- uted by the American Society for Horticultural Science. Potassium APS: © The American Phytopathological Society; from symbol: K; available to plants as the ion K+ Nutrient Deficiencies and Toxicities in Crop Plants, Nutrient functions W.F. Bennett (editor), 1993; used with permission. ¥ Unlike N and P, K does not form any vital organic PPI: Potash & Phosphate Institute; used with permission. compounds in the plant. However, the presence of USSL: USDA-ARS George E. Brown, Jr. Salinity Lab- K is vital for plant growth because K is known to be oratory, Riverside, CA; eucalyptus photos courtesy of an enzyme activator that promotes metabolism. Catherine M. Grieve.

32 Essential Nutrients Plant Nutrient Management in Hawaii’s Soils

PPI

Nitrogen deficient cucumber fruit is misshapen and chlorotic.

PPI

Nitrogen deficient potato plant (left) is chlorotic; plant at right is normal.

PPI

Severe nitrogen deficiency in citrus.

Nitrogen deficient corn; yellowing proceeds down the midrib of older leaves. Deficiency symptoms

PPI ¥ Stunted growth may occur because of reduction in cell division. ¥ Pale green to light yellow color (chlorosis) appearing first on older leaves, usually starting at the tips. Depending on the severity of deficiency, the chlorosis could result in the death and/or dropping of the older leaves. This is caused by the translocation of N from the older to the younger tissues. ¥ Reduced N lowers the protein content of seeds and vegetative parts. In severe cases, flowering is greatly reduced. ¥ N deficiency causes early maturity in some crops, which results in a significant reduction in yield and quality.

Nitrogen deficient soybean; lower leaves turn uniformly pale green, then yellow. N

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PPI

Phosphorus deficient sugarbeet plants are stunted, with dark green leaves.

PPI

N. Tamimi

Phosphorus deficient tomato leaves have purple interveinal tissue on their undersides. Phosphorus deficient corn; lower leaves become reddish-purple.

APS Deficiency symptoms ¥ Because P is needed in large quantities during the early stages of cell division, the initial overall symptom is slow, weak, and stunted growth. ¥ P is relatively mobile in plants and can be transferred to sites of new growth, causing symptoms of dark to blue-green coloration to appear on older leaves of some plants. Under severe deficiency, purpling of leaves and stems may appear. ¥ Lack of P can cause delayed maturity and poor seed and fruit development. Phosphorus deficient potato plants are stunted, with dark green leaves. P

34 Essential Nutrients Plant Nutrient Management in Hawaii’s Soils

PPI

R. L. Fox

Potassium deficient tomato leaves have chlorotic and necrotic spotting. Potassium deficient banana; older leaves become chlorotic, then necrotic, and the tip of the midrib bends downward.

PPI

Potassium deficient corn; margins of older leaves become chlorotic and necrotic.

PPI

Deficiency symptoms ¥ The most common symptom is chlorosis along the edges of leaves (leaf margin scorching). This occurs first in older leaves, because K is very mobile in the plant. ¥ Because K is needed in photosynthesis and the synthesis of proteins, plants lacking K will have slow and stunted growth. ¥ In some crops, stems are weak and lodging is common if K is deficient. ¥ The size of seeds and fruits and the quantity of their production is reduced.

Potassium deficient squash leaf edges become chlorotic and necrotic. K

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PPI

Calcium deficient celery; Calcium deficient tomato; young leaves are necrotic and the growing point dies. young leaves become twisted and cupped.

PPI

N. Tamimi

Calcium deficient corn leaves fail to unfold. Calcium deficient bean leaves have chlorotic and necrotic spots.

Deficiency symptoms ¥ Ca is not mobile and is not translocated in the ¥ Without adequate Ca, which in the form of cal- plant, so symptoms first appear on the younger cium pectate is needed to form rigid cell walls, leaves and leaf tips. The growing tips of roots and newly emerging leaves may stick together at the leaves turn brown and die. margins, which causes tearing as the leaves ex- ¥ Ca deficiency is not often observed in plants be- pand and unfurl. This may also cause the stem cause secondary effects of high acidity resulting structure to be weakened. from soil calcium deficiency usually limit growth, ¥ In some crops, younger leaves may be cupped precluding expressions of Ca deficiency symp- and crinkled, with the terminal bud deteriorating. toms. ¥ Buds and blossoms fall prematurely in some crops. Ca

36 Essential Nutrients Plant Nutrient Management in Hawaii’s Soils

PPI

Magnesium deficient soybean; interveinal chlorosis of older leaves.

PPI

Magnesium deficient sweetpotato leaves become reddish-purple.

Magnesium deficient corn; interveinal chlorosis of older leaves.

PPI Deficiency symptoms ¥ Because Mg is a mobile element and part of the chlorophyll molecule, the deficiency symptom of interveinal chlorosis first appears in older leaves. Leaf tissue between the veins may be yellowish, bronze, or reddish, while the leaf veins remain green. Corn leaves appear yellow-striped with green veins, while crops such as potatoes, tomatoes, soybeans, and cabbage show orange-yellow color with green veins. ¥ In severe cases, symptoms may appear on younger leaves and cause premature leaf drop. ¥ Symptoms occur most frequently in acid soils and soils receiving high amounts of K fertilizer or Ca.

Magnesium deficient tomato; interveinal chlorosis of older leaves. Mg

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APS

R. L. Fox

Sulfur deficient banana; young leaves are uniformly chlorotic.

R. L. Fox

Sulfur deficient sorghum; young leaves are uniformly chlorotic.

Sulfur deficient macadamia; young leaves are chlorotic.

PPI

Deficiency symptoms ¥ Younger leaves are chlorotic with evenly, lightly colored veins. In some plants (e.g., citrus) the older leaves may show symptoms first. However, deficiency is not commonly found in most plants. ¥ Growth rate is retarded and maturity is delayed. ¥ Plant stems are stiff, thin, and woody. ¥ Symptoms may be similar to N deficiency and are most often found in sandy soils that are low in organic matter and receive moderate to heavy rainfall.

Sulfur deficient tomato; young leaves are uniformly chlorotic. S

38 Essential Nutrients Plant Nutrient Management in Hawaii’s Soils

PPI

Boron deficient radish plants are stunted, with distorted leaves.

APS

Boron deficient tomato leaves are chlorotic, with some curling.

Boron deficient potato leaves have light brown edges; crinkling around the center of the leaf blade causes an upward-cupped shape; the plant’s growing point dies. Deficiency symptoms ¥ Generally, B deficiency causes stunted growth, first showing symptoms on the growing point and younger leaves. The leaves tend to be thick-

M. Nishina ened and may curl and become brittle. ¥ In many crops, the symptoms are well defined and crop-specific, such as: ¥ peanuts: hollow hearts ¥ celery: crooked and cracked stem ¥ beets: black hearts ¥ papaya: distorted and lumpy fruit ¥ carnation: splitting of calyx ¥ Chinese cabbage: midribs crack, turn brown ¥ cabbage, broccoli, and cauliflower: pith in hollow stem

Boron deficient papaya fruits develop bumps. B

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PPI

Copper deficient corn leaf tips bend and droop. Copper deficient lettuce leaves are stunted and cupped, with darker tissue near the petiole.

PPI

APS

Copper deficient tomato leaves are stunted and deformed.

Deficiency symptoms ¥ Reduced growth, distortion of the younger leaves, and possible necrosis of the apical meristem. ¥ In trees, multiple sprouts occur at growing points, resulting in a bushy appearance. Young leaves becomes bleached, and eventually there is defoliation and dieback of twigs. ¥ In forage grasses, young leaf tips and growing points are affected first. The plant is stunted and chlorotic.

Copper deficient onion leaves have white tips and twist in spirals or right angles. Cu

40 Essential Nutrients Plant Nutrient Management in Hawaii’s Soils

PPI

Chlorine deficient tomato;

leaf edges roll upward. APS

Chlorine deficient sugarbeet; young leaves are chlorotic.

Deficiency symptoms ¥ Chlorosis of younger leaves and wilting of the plant. ¥ Deficiency seldom occurs because Cl is found in the atmosphere and rainwater. Cl

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PPI

R. L. Fox

Iron deficient soybean; young leaves have interveinal chlorosis. Iron deficient macademia; young leaves have interveinal chlorosis.

PPI

Iron deficient gardenia; young leaves have interveinal chlorosis.

APS

Deficiency symptoms ¥ Interveinal chlorosis in younger leaves. The youngest leaves maybe white, because Fe, like Mg, is involved in chlorophyll production. ¥ Usually observed in alkaline or over-limed soils.

Iron deficient corn; young leaves have interveinal chlorosis. Fe

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APS

Manganese deficient tomato; Manganese deficient soybean; expanded young leaves have green veins young leaves have interveinal chlorosis. with interveinal chlorosis.

PPI

APS

Manganese deficient corn; young leaves are Manganese deficient orange; leaves develop a olive-green and slightly streaked. mottled pattern of light and dark green.

Deficiency symptoms ¥ Symptoms first appear as chlorosis in young tissues. ¥ In legumes, necrotic areas develop on the cotyledons, Unlike Fe chlorosis symptoms, in dicots Mn chlorosis a symptom known as marsh spots. shows up as tiny yellow spots. ¥ In monocots, greenish-grey specks appear at the lower base of younger leaves. The specks may eventually become yellowish to yellow-orange. Mn

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PPI

APS

Molybdenum deficient onion leaves show wilting and dieback.

PPI

Molybdenum deficient sugarbeet leaf has slight veining and pronounced necrotic spotting.

Molybdenum deficient grapefruit leaves have interveinal chlorosis.

PPI

Deficiency symptoms ¥ Deficiency symptoms resemble those of N because the function of Mo is to assimilate N in the plant. Older and middle leaves become chlorotic, and the leaf margins roll inwards. ¥ In contrast to N deficiency, necrotic spots appear at the leaf margins because of nitrate accumulation. ¥ Deficient plants are stunted, and flower formation may be restricted. ¥ Mo deficiency can be common in nitrogen-fixing legumes.

Molybdenum deficient tomato leaves have interveinal chlorosis. Mo

44 Essential Nutrients Plant Nutrient Management in Hawaii’s Soils

PPI

Zinc deficient mango shoots have shortened internodes, resulting in leaf rosetting.

ASHS

Zinc deficient corn. Young leaves have interveinal, chlorotic stripes on both sides of the midrib.

Zinc deficient avocado; young leaves have interveinal chlorosis. Deficiency symptoms ¥ Interveinal chlorosis occurs on younger leaves,

APS similar to Fe deficiency. However, Zn deficiency is more defined, appearing as banding at the basal part of the leaf, whereas Fe deficiency results in interveinal chlorosis along the entire length of the leaf. ¥ In vegetable crops, color change appears in the younger leaves first. The new leaves are usually abnormally small, mottled, and chlorotic. ¥ In citrus, irregular interveinal chlorosis occurs with small, pointed, mottled leaves. Fruit formation is significantly reduced. ¥ In legumes, stunted growth with interveinal chlorosis appears on the older, lower leaves. Dead tissue drops out of the chlorotic spots.

Zinc deficient onion leaves are twisted (right); plants at left are normal. Zn

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USSL

Boron toxicity in bean. Boron toxicity in soybean Plants are stunted and become yellow and necrotic. affecting newly opened leaves.

Elemental toxicity

USSL B

USSL

Boron toxicity in green onion.

Boron toxicity in salt-stressed eucalyptus (Eucalyp- tus camaldulensis). Reddish areas close to margins are early symptoms; necrotic leaf tips and margins are later symptoms of boron damage.

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APS

N. V. Hue N. V.

Manganese toxicity in soybean. Growth is stunted; leaves have necrotic spots and curl upward.

Aluminum toxicity in tomato. Mn Plants are stunted; developing leaves become white. Al Elemental toxicity

Color photographs for Chapter 15, Organic Soil Amendments for Sustainable Agriculture

Figure 15-1. Corn field with nitrogen deficiency. Figure 15-2. Effects of different composts on tomato seedlings when mixed with soil at 25 or 50% (volume).

APS

N.V. Hue N.V.

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Color photographs for Chapter 15 (continued)

Figure 15-3. Phosphorus deficiency symptoms in corn and guava.

R.L. Fox

N.V. Hue N.V.

Figure 15-4. Potassium deficiency affects tomato quality and corn maturity.

PPI

Figure 15-5. Potassium deficiency symptoms in soybean and alfalfa.

PPI

48 Essential Nutrients Plant Nutrient Management in Hawaii’s Soils

Calcium rophyll molecule, the deficiency symptom of symbol: Ca; available to plants as the ion Ca2+ interveinal chlorosis first appears in older leaves. Leaf tissue between the veins may be yellowish, Nutrient functions bronze, or reddish, while the leaf veins remain green. Ca has a major role in the formation of the cell wall ¥ Corn leaves appear yellow-striped with green veins, membrane and its plasticity, affecting normal cell while crops such as potatoes, tomatoes, soybeans, division by maintaining cell integrity and membrane and cabbage show orange-yellow color with green permeability. veins. Ca is an activator of several enzyme systems in pro- ¥ In severe cases, symptoms may appear on younger tein synthesis and carbohydrate transfer. ¥ leaves and cause premature leaf drop. Ca combines with anions including organic acids, ¥ Symptoms occur most frequently in acid soils and sulfates, and phosphates. It acts as a detoxifying agent ¥ soils receiving high amounts of K fertilizer or Ca. by neutralizing organic acids in plants. Ca is essential for seed production in peanuts. ¥ Sulfur Ca indirectly assists in improving crop yields by re- ¥ symbol: S; available to plants as the sulfate ion, SO 2Ð ducing soil acidity when soils are limed. 4 Nutrient functions Deficiency symptoms (p. 36) S is essential in forming plant proteins because it is Ca is not mobile and is not translocated in the plant, ¥ ¥ a constituent of certain amino acids. so symptoms first appear on the younger leaves and It is actively involved in metabolism of the B vita- leaf tips. The growing tips of roots and leaves turn ¥ mins biotin and thiamine and co-enzyme A. brown and die. S aids in seed production, chlorophyll formation, Ca deficiency is not often observed in plants because ¥ ¥ nodule formation in legumes, and stabilizing pro- secondary effects of high acidity resulting from soil tein structure. calcium deficiency usually limit growth, precluding expressions of Ca deficiency symptoms. Deficiency symptoms (p. 38) ¥ Without adequate Ca, which in the form of calcium ¥ Younger leaves are chlorotic with evenly, lightly pectate is needed to form rigid cell walls, newly colored veins. In some plants (e.g., citrus) the older emerging leaves may stick together at the margins, leaves may show symptoms first. However, defi- which causes tearing as the leaves expand and un- ciency is not commonly found in most plants. furl. This may also cause the stem structure to be ¥ Growth rate is retarded and maturity is delayed. weakened. ¥ Plant stems are stiff, thin, and woody. ¥ In some crops, younger leaves may be cupped and ¥ Symptoms may be similar to N deficiency and are crinkled, with the terminal bud deteriorating. most often found in sandy soils that are low in or- ¥ Buds and blossoms fall prematurely in some crops. ganic matter and receive moderate to heavy rainfall.

Magnesium Boron 2+ symbol: Mg; available to plants as the ion Mg Symbol: B. Available to plants as borate, H3BO3 Nutrient functions Nutrient functions ¥ The predominant role of Mg is as a major constitu- ¥ B is necessary in the synthesis of one of the bases ent of the chlorophyll molecule, and it is therefore for RNA formation and in cellular activities. actively involved in photosynthesis. ¥ B has been shown to promote root growth. ¥ Mg is a co-factor in several enzymatic reactions that ¥ B is essential for pollen germination and growth of activate the phosphorylation processes. the pollen tube. ¥ Mg is required to stabilize ribosome particles and ¥ B has been associated with lignin synthesis, activi- also helps stabilize the structure of nucleic acids. ties of certain enzymes, seed and cell wall forma- ¥ Mg assists the movement of sugars within a plant. tion, and sugar transport. Deficiency symptoms (p. 37) Deficiency symptoms (p. 39) ¥ Because Mg is a mobile element and part of the chlo- ¥ Generally, B deficiency causes stunted growth, first

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showing symptoms on the growing point and Iron younger leaves. The leaves tend to be thickened and symbol: Fe; available to plants as Fe2+, Fe3+ may curl and become brittle. Nutrient functions In many crops, the symptoms are well defined and ¥ Fe is essential in the heme enzyme system in plant crop-specific, such as: ¥ metabolism (photosynthesis and respiration). The en- peanuts: hollow hearts ¥ zymes involved include catalase, peroxidase, cyto- celery: crooked and cracked stem ¥ chrome oxidase, and other cytochromes. beets: black hearts ¥ ¥ Fe is part of protein ferredoxin and is required in papaya: distorted and lumpy fruit ¥ nitrate and sulfate reductions. carnation: splitting of calyx ¥ ¥ Fe is essential in the synthesis and maintenance of Chinese cabbage: midribs crack, turn brown ¥ chlorophyll in plants. cabbage, broccoli, and cauliflower: pith in ¥ ¥ Fe has been strongly associated with protein metabo- hollow stem lism. Copper Deficiency symptoms (p. 42) symbol: Cu; available to plants as the ion Cu++ ¥ Interveinal chlorosis in younger leaves. The youngest leaves maybe white, because Fe, like Mg, is in- Nutrient functions volved in chlorophyll production. Cu is essential in several plant enzyme systems in- ¥ Usually observed in alkaline or over-limed soils. volved in photosynthesis. ¥ Cu is part of the chloroplast protein plastocyanin, ¥ Manganese which forms part of the electron transport chain. symbol: Mn; available to plants as Mn2+, Mn3+ ¥ Cu may have a role in the synthesis and/or stability of chlorophyll and other plant pigments. Nutrient functions Mn primarily functions as part of the plant enzyme Deficiency symptoms (p. 40) ¥ system, activating several metabolic functions. It is Reduced growth, distortion of the younger leaves, ¥ a constituent of pyruvate carboxylase. and possible necrosis of the apical meristem. Mn is involved in the oxidation-reduction process In trees, multiple sprouts occur at growing points, ¥ ¥ in photosynthesis. resulting in a bushy appearance. Young leaves be- Mn is necessary in Photosystem II, where it partici- come bleached, and eventually there is defoliation ¥ pates in photolysis. and dieback of twigs. Mn activates indole acetic acid oxidase, which then In forage grasses, young leaf tips and growing points ¥ ¥ oxidizes indole acetic acid in plants. are affected first. The plant is stunted and chlorotic. Deficiency symptoms (p. 43) Chlorine ¥ Symptoms first appear as chlorosis in young tissues. symbol: Cl; available to plants as the chloride ion, ClÐ Unlike Fe chlorosis symptoms, in dicots Mn chlorosis shows up as tiny yellow spots. Nutrient functions In monocots, greenish-grey specks appear at the Cl is essential in photosynthesis, where it is involved ¥ ¥ lower base of younger leaves. The specks may even- in the evolution of oxygen. tually become yellowish to yellow-orange. Cl increases cell osmotic pressure and the water con- ¥ In legumes, necrotic areas develop on the cotyledons, tent of plant tissues. ¥ a symptom known as marsh spots. ¥ Cl is found in many bacteria and fungi. Cl reduces the severity of certain fungal diseases, ¥ Molybdenum e.g., take-all disease of wheat. symbol: Mo; available to plants as molybdate, MoO4 Deficiency symptoms (p. 41) Nutrient functions Chlorosis of younger leaves and wilting of the plant. ¥ Mo is a necessary component of two major enzymes Deficiency seldom occurs because Cl is found in the ¥ ¥ in plants, nitrate reductase and nitrogenase, which atmosphere and rainwater.

50 Essential Nutrients Plant Nutrient Management in Hawaii’s Soils

are required for normal assimilation of N. ¥ Mo is required by some soil microorganisms for nitrogen fixation in soils. Deficiency symptoms (p. 44) ¥ Deficiency symptoms resemble those of N because the function of Mo is to assimilate N in the plant. Older and middle leaves become chlorotic, and the leaf margins roll inwards. ¥ In contrast to N deficiency, necrotic spots appear at the leaf margins because of nitrate accumulation. ¥ Deficient plants are stunted, and flower formation may be restricted. ¥ Mo deficiency can be common in nitrogen-fixing legumes.

Zinc symbol: Zn; available to plants as Zn++ Nutrient functions ¥ Zn is required in the synthesis of tryptophan, which in turn is necessary for the formation of indole acetic acid in plants. ¥ Zn is an essential component of several metallo-enzymes in plants (variety dehydorgenases) and therefore is necessary for several different function in plant metabolism. ¥ The enzyme carbonic anhydrase is specifically acti- vated by Zn. ¥ Zn has a role in RNA and protein synthesis. Deficiency symptoms (p. 45) ¥ Interveinal chlorosis occurs on younger leaves, similar to Fe deficiency. However, Zn deficiency is more defined, appearing as banding at the basal part of the leaf, whereas Fe deficiency results in interveinal chlorosis along the entire length of the leaf. ¥ In vegetable crops, color change appears in the younger leaves first. The new leaves are usually abnormally small, mottled, and chlorotic. ¥ In citrus, irregular interveinal chlorosis occurs with small, pointed, mottled leaves. Fruit formation is significantly reduced. ¥ In legumes, stunted growth with interveinal chlorosis appears on the older, lower leaves. Dead tissue drops out of the chlorotic spots.

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The nitrogen cycle Still other soil microorganisms oxidize ammonium + Ð (NH4 ) to nitrate (NO3 ) in a two-step process called + Ð N inputs nitrification. NH4 is converted to nitrite (NO2 ) by Atmospheric nitrogen is converted to forms available Nitrosomonas bacteria: to plants through natural atmospheric activity, indus- 2HN + + 3O = 2NO Ð + 2H O + 4H+ trial processes, and biological fixation. 4 2 2 2 Atmospheric nitrogen (N ) is a gas at normal at- 2 Then nitrite is converted to nitrate by Nitrobacter bac- mospheric temperatures and is not directly available to teria: higher plants. Before plants can use N, it must be con- Ð + Ð Ð verted to nitrate (NO3 ) or ammonia (NH4 ). Some N is 2NO2 + O2 = 2 NO3 converted to usable forms when lightning’s energy

breaks the strong bond of the N2 gas molocule. The Nitrogen is applied as fertilizer in organic forms resulting oxide forms of N can acquire hydrogen from as animal manures, composts, and green manure crops. + atmospheric moisture and be precipitated as NH4 in Inorganic N fertilizers are available in liquid, granular, rainfall. and slow-release formulations. Ammonium-N fertiliz- + N2 can also be converted industrially. The gas ers delay leaching because NH4 is held on the soil + Ð molocule is destabilized by putting air under great heat exchange complex. When NH4 is converted to NO3 , and pressure and injecting a hydrogen source (usually N is more subject to leaching loss. Urea fertilizer + natural gas, or methane) to form ammonium (NH3), leaches readily until it is converted in the soil to NH4 which is the building-block of the various inorganic N by the naturally occuring enzyme urease, usually within fertilizers. Because the process uses so much energy— a few days of application. usually fossil-fuel based—inorganic N fertilizers are relatively expensive. N losses

N2 can be converted by rhizobia bacteria that live Nitrogen is removed when crops are harvested, and it in symbiosis with legumes. The rhizobia use enzymatic is also lost by leaching of nitrate ions during periods processes to break the N2 bond. In addition to rhizobia, of heavy rain or over-irrigation. Most soils have only

there are other microorganisms that can convert N2 to small amounts of positive electrical charge on their ex- Ð NO3 , either in symbiosis (e.g., Frankia, Anabaena) or change complex with which to hold the negatively Ð in free-living forms (e.g., Azotobacter, various actino- charged nitrate ion. If NO3 is not taken up by plants, it mycetes). can be leached below the root zone and into Organic nitrogen compounds are converted to groundwaters. Careful planning of irrigations and the plant-available forms when sources such as animal amount and timing of N fertilizer applications is nec- manures and plant residues are naturally degraded in essary to avoid groundwater contamination. Monitor- the soil in a process known as mineralization. First, ing the crop water balance can help to conserve irriga- proteins are transformed by microorganisms to their tion water and fertilizer and protect the environment. constituent amines and amino acids by aminization: Nitrogen can be lost to the atmosphere by volatil- ization. Under alkaline soil conditions, ammonia can Protein + H O = R-NH + CO + energy 2 2 2 escape from the soil surface. Under anaerobic conditions, denitrification occurs when soil bacteria reduce Then the amines and amino acids are ammonified by NO Ð to N gas and nitrous oxide gas (N O), which es- other organisms: 3 2 2 cape to the atmosphere.

R-NH2 + H2O = NH3 + R-OH + energy + Ð NH3 + H2O = NH4 + OH

52 Essential Nutrients Plant Nutrient Management in Hawaii’s Soils

The nitrogen cycle

Nitrogen fixation by lightening

Oxides of nitrogen dissolved in rainwater

Atmospheric

nitrogen (N2)

Loss

Nitrogen fixation Animal use by microbes

Crop residues and other organic wastes

Azotobacter Rhizobium

Denitrification Organic nitrogen (in wet soils) Plant uptake in -NH groups N , N O, NO 2 2 2 (immobilization)

Ammonification

Nitrification NO NO + 3 2 NH4

NH + fixation Microbial use 4 Leaching by clays (immobilization)

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The phosphorus cycle

Animal use

P fertilizer Plant residues

Erosion losses

Immobilization by plant intake

Mineralization Solid Organic P inorganic P Soil solution P in humus compounds Immobilization by microbes

Adsorbed P

The phosphorus cycle P inputs are found, while in neutral, calcareous soils, octacal- Plants absorb P from the soil solution as phosphate in cium phosphate and apatite are found. the form H PO Ð at soil pH below 7.2 and HPO 2Ð at 2 4 4 Mineralization by microorganisms of P associated pH above 7.2. Phosphate dissolved in the soil water with organic matter is important in maintaining levels exists in an equilibrium between P in solution, labile P, of P in the soil solution. and nonlabile P : P losses Soil solution P Labile P Nonlabile P Phosphorus is removed in harvested crop materials, but in lesser amounts than nitrogen or potassium. Soil ero- Labile P is plant-available P that is adsorbed loosely sion can be a major source of P loss, because P is held by the soil but can be desorbed into solution. Nonlabile tightly by soils. If soil erosion in not controlled, phos- P has formed stable and relatively insoluble compounds phate pollution of the environment can result. Phos- with the soil, and it is not available to plants. phate can be strongly “fixed” (adsorbed) by soils. Un- Decomposition of soil minerals through weather- der acidic soil conditions, P combines with iron and ing releases P into the soil solution, but the process is aluminum. Under neutral or alkaline soil conditions, P gradual. In acid soils, iron and aluminum phosphates combines with calcium.

54 Essential Nutrients Plant Nutrient Management in Hawaii’s Soils

The potassium cycle

Animal use

Crop residues and manures K fertilizer Erosion losses

Plant uptake

Soil solution K

Exchangeable K

Fixation Release

Leaching Fixed K

K minerals The potassium cycle K inputs K losses Potassium is released into the soil solution from clay Since plants take up large amounts of K, its removal in minerals. The quantities and rates of release depend on harvested crops can be significant. Leaching of K can the soil mineralogy, rainfall, and temperature, and can occur, particularly in coarse-textured soils, if fertiliz- also be influenced by crop management. Most soils in ers and irrigation water are applied carelessly. K losses Hawaii have only small quantities of potassium-bear- with eroded soil are not usually serious and can be mini- ing clay minerals, due to intense weathering. mized with good soil management practices. Inorganic fertilizer K is sold in various formula- Under conditions where K has been severly de- tions including liquid, granular, and slow-release. Rela- pleted from the soil minerals, K fixation can occur. K tively small amounts of K become available from de- can be fixed in the structural lattices of 2:1 clay miner- composition of manures and crop residues. Certain well als, such as illite and vermiculite. Fixed K is not readily waters used for irrigation in Hawaii have relatively high available to plants, but may be gradually released for amounts of K. plant uptake.